Method for manufacturing Ni-Al alloy anode for fuel cells using nickel powders

ABSTRACT

In a method for manufacturing Ni—Al alloy anode for fuel cells, in which, using nickel powders, Ni powders are mixed with Ni—Al alloy powders, which are hardly sintered in themselves, to assist a sintering of Ni—Al alloy, whereby Ni—Al alloy anode can be manufactured simply, economically and compatibly with mass production even by a conventional manufacturing process for an electrode.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for manufacturing Ni—Al alloyanode for fuel cells, more particularly to a method for manufacturingNi—Al alloy anode for fuel cells, in which, using nickel powders, Nipowders are mixed with Ni—Al alloy powders, which are hardly sintered inthemselves, to assist a sintering of Ni—Al alloy, whereby Ni—Al alloyanode can be manufactured simply, economically and compatibly with massproduction even by a conventional manufacturing process for anelectrode.

Generally known in the art, fuel cells are a generator directlytransforming chemical energy fuel has into electric energy. Fuel cellsare classified into various kinds of fuel cells including MoltenCarbonate Fuel Cell (MCFC), Polymer Electrolyte Membrane Fuel Cell(PEMFC), Solid Oxide Fuel Cell (SOFC) and the like. MCFC is a fuel cellusing molten carbonate as an electrolyte and consists of the key spareparts of a cathode, an electrolyte and supporter, and an anode.

In case of a high temperature fuel cell operating at a temperature ofabove 500° C., such as MCFC and SOFC, Ni is generally used as anelectrode material. For example, in MCFC, porous Ni is used as an anodeand NiO (oxidized Ni) is used as a cathode. Also, in SOFC, a cermet inwhich Ni is mixed with electrolytic material such as zirconia or ceriaand the like is used as an anode.

A serious problem in the anode where an oxidation reaction of fuel isgenerated is that, under an operating condition of high temperature andheavy load of above 2 kg/cm², a sintering and a creep are caused so thatporosity is reduced and a micro-structural deformation such as shrinkageis generated, degrading performance thereof.

That is to say, Ni electrode adapted to high temperature fuel cell ismanufactured to have porous structure in order to enlarge reactive areaof the electrode and to provide a gas passage way, but, if Ni electrodeis used at a high temperature for a long time, it has defects in thatsurface area and reaction rate thereof are reduced. Also, if a fuel cellstack many sheets of unit cells are laminated one after another isoperated for a long time, a creep is caused in the porous Ni electrodeby a load of the fuel cell, causing a defect of performance reduction.

To solve this problem, there has been proposed a method for improvingresistance to sintering and a creep of the nickel electrode, wherein 10wt % Cr is added to Ni to form an intermetallic compound between Ni andCr, or oxides such as Cr₂O₃, LiCrO₂ and so forth are formed on thesurface of nickel electrode.

It has been reported that the modulus of strain of conventional Ni-10%Cr anode by creep is below 5%, but LiCrO₂ formed on the surface isdissolved in the electrolyte to weaken resistance to sintering and creepwhen operated long time. As a result, in order to improve a feature ofcreep, after the middle of 1980s, there have been studied a method ofoxide dispersion strengthened (ODS) in which metal oxide includingalumina is dispersed over the Ni electrode, and other methods usingNi—Al or Ni—Cr alloy as an anode, the alloy containing small quantitiesof Al or Cr that is preferentially oxidized relative to Ni.

ODS method had an effect in improvement of creep feature, but also had alimit in manufacturing an electrode having proper mechanical strengthand electric conductivity.

Meanwhile, the method using an alloy electrode is a method which has thesame concept as the ODS method and which is proposed to solve theproblem in degradation of mechanical strength by previously dispersingAl or Cr, which will be oxidized during a manufacturing process of anelectrode or during an operation, over the Ni substrate so that theproduced oxides distribute over inside and outside of the substrate andthe surface thereof. Known as best material among the alloy electrodesis Ni—Al alloy electrode, which has below 0.5% of the creep strain rateso that, even in Im², the size of commercial electrode, an increase ofcontact resistance is very slight.

However, Ni—Al alloy electrode has problems in that its price is higherthan the existing material and in that it is not easily sintered by aconventional manufacturing process of electrode.

Accordingly, it has been required a method for manufacturing a new alloyanode for electrode of fuel cells, which is easy to be sintered andmanufactured at low cost, providing a simple and economical process, andwhich has excellent working property advantageous for scale-up and massproduction.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve theabove-mentioned problems occurring in the prior art, and an object ofthe present invention is to provide to a method for manufacturing Ni—Alalloy anode for fuel cells, in which, using nickel powders, Ni powdersare mixed with Ni—Al alloy powders, which are hardly sintered inthemselves, to assist a sintering of Ni—Al alloy, whereby Ni—Al alloyanode can be manufactured simply, economically and compatibly with massproduction even by a conventional manufacturing process for anelectrode.

Another object of the present invention is to provide Ni—Al alloy anodefor fuel cells, which are manufactured by the method so that structuralstability thereof is excellent while maintaining reactive activitythereof as it is.

In order to accomplish the above object, there is provided a method formanufacturing Ni—Al alloy anode for fuel cells using nickel powders,wherein the nickel powders are mixed as sintering aids into Ni—Al alloypowders.

According to the method for manufacturing Ni—Al alloy anode for fuelcells using nickel powders, mixing ratio of Ni—Al alloy powders to Nipowders is 30:70 to 70:30.

According to the method for manufacturing Ni—Al alloy anode for fuelcells using nickel powders, mixing ratio of Ni—Al alloy powders to Nipowders is 40:60 to 60:40.

Ni—Al alloy anode for fuel cells of the present invention ismanufactured by the method described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view showing a structure of Ni—Al alloy anode forfuel cells manufactured using nickel powders by the present invention;and

FIG. 2 is a graphical representation showing performance of unitelectrode using Ni—Al alloy anode for fuel cells manufactured usingnickel powders by the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings.

Since it is difficult for Ni—Al alloy powders to be sintered at hightemperature, mass production of Ni—Al alloy anode is very difficult.

The manufacturing method of Ni—Al anode according to the presentinvention is a method in which Ni powders are mixed to facilitate asintering of Ni—Al alloy anode. Herein, Ni powders should be added bycertain quantity to the extent that they assist only sintering of Ni—Alalloy powders. To maintain features of the existing Ni—Al alloy anode asit is, a microstructure of the anode should be controlled as to have amicrostructure as shown in FIG. 1.

That is, a feature resistant to a creep the existing Ni—Al alloy anodehas can be expected only when Ni—Al alloy powders have a microstructurewith 3-D network as shown in FIG. 1.

In the present invention, a resistant feature to the creep is changedaccording to a volume ratio (mass ratio) of Ni—Al alloy powders to addedNi powders. Beyond a specific volume ratio that Ni—Al alloy powders forma structure of 3-D network, the resistant feature to creep becomes toabruptly increase.

The microstructure of the anode is determined in consideration of thesecuring of reaction area and gas transfer passage way, distribution inMCFC and electric conductivity, so that a thickness of about 0.8 mm, aninitial porosity of above 50%, an average pore size of 3˜5 μm aretypically provided. The anode with such structure is generallymanufactured by a tape casting method, which is advantageous forscale-up. Since Ni—Al alloy anode for fuel cells can be manufactured bya conventional method, the detailed explanation of the method will beomitted.

Hereinafter, a construction and an effect thereby of the presentinvention will be described in detail with reference to the followingembodiment. Although the following embodiment illustrates contents ofthe present invention, the present invention should not be limited tothe embodiment.

Embodiment 1

In this embodiment, Ni—Al alloy anode is manufactured by the followingmethod.

Ni—Al alloy powders and Ni powders were mixed with each other in avolume ratio of 50:50, and then sintered at 1000° C. for more than 3hours. The porosity and pore size of the electrode were proved to besimilar to those of the existing Ni—Cr anode.

The manufactured anode was adapted to the unit cell and operated at 650°C.

FIG. 2 shows a result of measuring performance of the unit cell to whichthe anode is adapted, the anode being manufactured by mixing Ni—Al alloypowders and Ni powders in a volume ratio of 50:50. The unit cellaccording to the present invention maintains a performance of above 0.8Vat a load of 150 mA/cm² without the degradation of performance for morethan 3000 hours. However, near on 3300 hours, the measuring of cellperformance was stopped while the temperature was raised by about 800°C. due to a disorder of thermocouple.

Generally, performance degradation of cell caused by a structuralinstability of the anode involves an increase of internal resistance(IR). However, as shown in FIG. 2, the anode manufactured by the presentinvention maintains value of IR as it is, which indicates that the anodeof the present invention has a structural stability superior to that ofthe existing anode.

As described above, the present invention provides a method formanufacturing Ni—Al alloy anode for fuel cells, in which, using nickelpowders, Ni powders are mixed with Ni—Al alloy powders, which are hardlysintered in themselves, to assist a sintering of Ni—Al alloy, wherebyNi—Al alloy anode can be manufactured simply, economically andcompatibly with mass production even by a conventional manufacturingprocess for an electrode.

Also, the method for manufacturing Ni—Al alloy anode for fuel cellsusing nickel powders utilizes the existing manufacturing process ofelectrode based on Ni as it is, so that Ni—Al alloy anode can bemanufactured economically and compatibly with scale-up of the anode.

Although preferred embodiments of the present invention have beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method for manufacturing Ni—Al alloy anode for fuel cells usingnickel powders, wherein the nickel powders are mixed as sintering aidsinto Ni—Al alloy powders.
 2. A method for manufacturing Ni—Al alloyanode for fuel cells using nickel powders as claimed in claim 1, whereinmixing ratio of Ni—Al alloy powders to Ni powders is 30:70 to 70:30. 3.A method for manufacturing Ni—Al alloy anode for fuel cells using nickelpowders as claimed in claim 1, wherein mixing ratio of Ni—Al alloypowders to Ni powders is 40:60 to 60:40.
 4. A Ni—Al alloy anode for fuelcells, manufactured by the method according to claim
 1. 5. A Ni—Al alloyanode for fuel cells, manufactured by the method according to claim 2.6. A Ni—Al alloy anode for fuel cells, manufactured by the methodaccording to claim 3.